EP1835990A1

EP1835990A1 - Device and method for preparing a homogeneous mixture consisting of fuel and oxidants

Info

Publication number: EP1835990A1
Application number: EP05850151A
Authority: EP
Inventors: Stefan Kah; Andreas Lindermeir; Robert Engel
Original assignee: Webasto SE
Current assignee: Enerday GmbH
Priority date: 2005-01-14
Filing date: 2005-12-13
Publication date: 2007-09-26
Anticipated expiration: 2025-12-13
Also published as: EA010875B1; JP2011174696A; ATE471206T1; KR20070100796A; DE102005001900A1; EA200701508A1; JP2008527300A; AU2005325030A1; KR100975101B1; CN101142016B; CA2594766A1; US20080134580A1; DE102005001900B4; EP1835990B1; WO2006074622A1; DE502005009771D1; CN101142016A

Abstract

A device for providing a homogenous mixture of fuel and oxidant including an arrangement ( 5 ) for feeding liquid fuel to an evaporator, an arrangement ( 4 ) for feeding gaseous oxidant into a mixing zone ( 12 ) downstream of the evaporator, and a reaction zone downstream of the mixing zone in which a packed structure ( 3 ) is arranged. The packed structure can be a ceramic cylindrical molding having a diameter in the range 25 to 35 mm and an axial length in the range 15 to 50 mm or it can have flow conduits that are square in cross-section and have a cell density in the range 400 to 1200 cpsi.

Description

Apparatus and method for providing a homogeneous mixture of fuel and oxidant

The invention relates to a device for providing a homogeneous mixture of fuel and oxidant, comprising means for supplying liquid fuel to an evaporator, means for supplying gaseous oxidant in a mixing zone downstream of the evaporator and a reaction zone downstream of the mixing zone.

The invention further relates to a method for providing a homogeneous mixture of fuel and oxidant, comprising the steps of: supplying liquid fuel to an evaporator, supplying gaseous oxidant and vaporized fuel into a mixing zone downstream of the evaporator, mixing oxidant and Fuel in the mixing zone and introducing the resulting mixture in the mixing zone in a reaction zone

Liquid fuels such as diesel, fuel oil, gasoline and kerosene are now the most important source of energy to generate heat, mechanical work and electricity. This is z. B. used in combustion in automobiles, parking heaters and household burners. In fuel cell systems, however, liquid hydrocarbons are not completely burned, but z. B. converted into hydrogen by partial oxidation reactions. Both reaction types have in common that the liquid fuel must first be transferred to the gas phase and mixed in a mixing chamber with air, to be subsequently converted in the reaction chamber. In this case, the most homogeneous possible fuel-air mixture is sought, since with increasing homogeneity, the proportion of undesirable emissions in the form of, for example, soot, NO _x and CO can be reduced. CO as a reaction product is desirable as part of a reforming process while undesirable as a product of a combustion process.

In modern burner types and oxidation reactors, the mixing and reaction chambers are often separated from one another by a flame arrestor lock, so that when using self-igniting fuels, the oxidation reaction does not already begin within the mixing chamber and has a disadvantageous effect there.

The evaporation of the liquid fuel can be carried out by different methods:

According to z. B. DE 39 146 11 C2, the fuel evaporation takes place in vehicle heaters on the surface of metal fiber webs or similarly structured surfaces, such as fabrics. The liquid fuel is thereby applied to a hot metal fiber fleece, distributed there and then evaporated. After evaporation, the fuel is mixed with air in a mixing chamber and homogenized. The combustion air supply into the combustion chamber takes place stepwise through a plurality of air inlet holes in the combustion chamber wall. Flames can then form at these points and due to heat conduction, radiation and con- ultimately provide the necessary heat to maintain the evaporation process.

However, there are disadvantages in connection with fuel vaporization by means of metal fiber webs. Own measurements showed in DieseiVerdampfung very high surface temperatures (up to 1100 ⁰ C) on a metal fiber fleece, that is, in particular above 400 ⁰ C., from this temperature to an appreciable extent Crackreak- functions occur and soot formation is recorded. The disadvantage is that the fuel can only come in contact with air at the fleece surface and thus can oxidize. In addition, due to the stepped air supply, the largest proportion of the combustion air is supplied significantly after fleece evaporation. The correspondingly low air ratio at the nonwoven surface leads to the formation of deposits on the evaporator, which make a stable operation of the same and reduce its service life. There are reaction conditions which are comparable to those of the steam cracking process and result in soot formation, which will be briefly explained below.

Steam cracking is a thermal cracking process in the petrochemical industry carried out in the presence of water vapor. This is usually naphtha, a light gasoline fraction, with steam at ca. 800-1400 ⁰ C within from the outside fired cracking coils non-catalytically cracked to produce reactive low molecular weight compounds such as ethene and propene. Other technically interesting by-products formed include aromatics (benzene, toluene, xylene). The aim of steam cracking is to use what is needed in the chemical industry produce short-chain olefins. As can be seen in FIG. 4, however, these are only formed at high temperatures. The formation of ethane from ethane is favored above about 700 ⁰ C, and the formation of ethyne from ethene takes place above 1200 ⁰ C. Therefore, it is understandable that one uses for the production of ethene and propene temperatures of 800 to 900 ⁰ C (medium temperature pyrolysis), while the formation of acetylene takes place at temperatures above 1300 ⁰ C (high-temperature pyrolysis). It can also be seen from FIG. 4 that the hydrocarbons preferably decompose into the elements C (carbon black) and H. To reduce the extent of this undesirable subsequent reaction, the reaction mixture after the optimum reaction time (0, 2 to 0, 5 s) as fast as possible (0.1 s) must be cooled so that the rate of formation of these undesirable products goes to zero. Thus, the product composition is controlled not by the thermodynamic equilibrium but by the slow kinetics.

However, the compounds mentioned are very reactive in the operating conditions prevailing in the steam cracker and tend to condensation and polymerization reactions, so that ultimately carbon black is also formed, which occupies the reaction tubes, the corresponding heat transfer through them deteriorates and thus an exchange of the cracking tube snakes required at regular intervals. The by-product water vapor is added to ensure a good radial heat distribution in the pipe, by dilution to lower the partial pressure, favoring the formation of the cracking products and a conversion of the previously to ensure formed, undesirable high molecular weight compounds (coke).

Cracking reactions can also be catalysed heterogeneously, for. B. by surface metal atoms, so-called active centers. In C. H . Bartholomew, Applied Catal. A gene. 212 (2001) 17-60 is the catalytic effect of z. B. Nickel, iron, cobalt in the steam reforming of methane and other hydrocarbons described.

If correspondingly high temperatures are to be found in the vapor phase in the vaporiser, pyrolysis reactions continue there. At catalytic surfaces (fleece, evaporator or combustion chamber wall, glow plug) previously formed soot particles or carbon black precursors can accelerate this process of homogeneous soot formation.

Soot formed in an evaporator or combustion chamber can adversely affect the evaporator and burner properties, so that higher emissions of soot, CO, hydrocarbons, smoke, aerosols and polycyclic aromatics can arise. Thus, the regular use of regeneration strategies is required to achieve elimination of the soot from the evaporator chamber or combustion chamber. The Rußabbau usually takes place by burning and thus requires a corresponding design of the combustion chamber. Since this process is material transport-controlled at correspondingly high temperatures, high contact times of soot and oxidizing agent in the combustion chamber are required, which can result in high combustion chamber volumes. Furthermore, unusually high temperatures can occur on the combustion chamber surfaces whose material and material properties affect the properties and reduce the service life.

The temperatures of up to 1100 ^° C. measured on the metal fiber webs and their surroundings also place very high demands on the material resistance of the metal fibers and the neighboring components (eg glow plug, mixing chamber).

In contrast to KraftStoffVerdampfung means metal fiber nonwovens, the "cold flame principle" according to EP 1 102 949 Bl based on a fuel evaporation in a ca. 300 ⁰ C preheated air flow. The oxidation reactions that start are equilibrium reactions with oxygen conversions of less than 20%, so that the gas mixture produced has an outlet temperature of approx. 480 ⁰ C has.

Cold-flame evaporators have the disadvantage that the evaporator air by means of another system component (eg., Electric heating, burner) to temperatures of approx. 300 to 500 ⁰ C must be heated so that the cold-flame reaction (low-temperature oxidation) can use. Thus, the spontaneous auto-ignition of the fuel-air mixture, which occurs at higher temperatures, can be avoided. Nevertheless, the air preheating is detrimental to a quick cold start and makes dynamic operation difficult.

The fuel-air mixtures generated by means of evaporators or nozzles can prevent spontaneous self-ignition in the evaporator by flashback between evaporator and downstream reaction space (combustion, partial oxidation) are preserved, which increases the complexity.

It is an object of the present invention to provide an apparatus and a method for providing a homogeneous mixture of fuel and oxidizer based on nonwoven vaporization, so that the disadvantages of the prior art are at least partially overcome.

The invention builds on the generic device in that a packed structure is arranged in the reaction zone. At the surface and / or within the packed structure, oxidation reactions take place which, due to their exotherm, keep the packed structure at operating temperature. Part of the heat released is used in the evaporator zone downstream of the evaporator for self-sustaining the evaporation process; the rest of the heat energy is removed convectively with the product stream. Preferably, partial oxidation of the particular fuel component (C _x H _y ) takes place, which may otherwise cause soot formation, for example, aromatics and room temperature liquid long chain hydrocarbons C _x H _y at x> 4.

Usefully, it is provided that the packed structure is formed as a ceramic cylindrical shaped body having a diameter between 25 and 35 mm and an axial length between 15 and 50 mm in length. The packed structure thus fits in its dimensions to the compact

Structure of the device according to the invention. The diameter is for example 30mm, and the axial length is for example at 20mm. In addition to the use of ceramic materials, such as cordierite, metallic packed structures are possible. When using ceramic materials, for example, oxides of silicon, aluminum, alkali metals (for example sodium), alkaline earth metals (for example magnesium), heavy metals (for example barium) rare earths (for example yttrium ) or mixtures thereof in question. A preferred ceramic is Cordierite, for example. Furthermore, it is also possible to use non-oxidic ceramics, for example carbides (for example silicon carbide) or nitrides. Packed structures of metallic materials are usually made of wound metal foils (eg FeCrAlloy steel). The stacking of metal mesh, metal mesh or the like into a packed structure is also possible.

Usefully, it is envisaged that the packed structure has square flow channels in cross section with a cell density between 400 and 1200 cpsi. In addition to square flow channels, hexagonal, triangular, round and wavy flow channels are also possible. The channels may be parallel or random (similar to a sponge). In addition to the many forms that can have packed structures, for example, round, rectangular, "racetrack", the design options with regard to the cell densities are different. For example, cell densities in the range of 400 cpsi (cells per square inch) are advantageous, while cell densities up to approx. 1200 cpsi are currently possible.

Usefully, it is provided that the surface of the packed structure at least partially a coating as Catalyst. For example, a noble metal coating can be provided. The surface of the flow channels of the packed structure can be increased by orders of magnitude and catalytically activated by coating (eg washcoat: layer thicknesses of a few μm). This makes it possible to carry out catalytic reactions at technically relevant reaction rates. The selectivity to the desired partial oxidation products (eg CO and H ₂ ) can be increased by using a suitable catalytically active packed structure. The latter can be used for catalytic hydrogen production (partial oxidation, autothermal reforming, steam reforming). Typical washcoat materials are oxides of aluminum, silicon, titanium. Common catalysts are ele- metals, such as Pt, Pd, Ni, etc.

It can also be provided that the reaction zone is followed by a homogenization zone. The products emerging from the packed structure, for example residual short-chain hydrocarbons, hydrogen, carbon monoxide, water, carbon dioxide, can be worked much easier in this, compared to the fuel / air Gemisoh before the packed structure to a homogeneous mixture become. This is favored, among other things, by the significantly larger diffusion coefficients of the small molecular species (for example hydrogen) produced compared to the long-chain feed components and the higher flame speeds. Due to the significantly higher temperatures of the components produced, a further improvement of the mass transfer and thus of the homogeneity takes place. The tendency to soot is, compared to the mixed chamber, due to the low concentration of hydrocarbons low.

Furthermore, it is usefully provided that the homogenization zone is followed by a further reaction zone. The resulting Produktgemiseh can z. B. be further oxidized by adding an additional oxidant stream. As a result, air figures can be achieved down to the area of combustion.

The device according to the invention is further developed in a useful manner in that it is operable in the mixing chamber with a defined air ratio below 0, 5. In this way, the device can be operated without a spontaneous ignition occurs. Thus, the proportion of oxidation reactions and, indirectly, the extent of endothermic cracking reactions in the mixing chamber can be kept low. Broadly ignited mixtures can be deliberately extinguished.

Furthermore, the device is advantageously further developed in that it can be operated in an evaporation zone arranged between the evaporator and the mixing zone at temperatures which do not or only insignificantly exceed temperatures corresponding to the boiling end of the fuel used. The final boiling point is diesel fuel at about 360 ⁰ C. If this is not exceeded, the proportion of thermal cracking products which lead to soot formation is correspondingly low. Due to the low temperatures, there is a minimal tendency for spontaneous self-ignition of the fuel / air mixture. The low temperatures can cause that kinetic limitation results in less carbon black than would be expected from thermodynamic equilibrium calculations.

Furthermore, it is preferred that the evaporation zone and the mixing zone have a volume such that mean residence times of the fuel-oxidizer mixture are in the order of the reaction times of oxidation reactions. As a result, there is a minimal tendency for spontaneous self-ignition of the fuel / air mixture. The short residence times can lead to less soot being formed by the short contact times than would be expected after thermodynamic equilibrium calculations.

The invention is based on the generic method in that a packed structure is arranged in the reaction zone through which the entire mixture or already formed reaction products are passed. In this way, the advantages and peculiarities of the device according to the invention are also implemented in the context of a method. This also applies with regard to the preferred embodiments of the device according to the invention or the resulting preferred method features.

The invention is based on the finding that by providing a packed structure, in particular in combination with further features of the device, disadvantages of the prior art can be at least partially overcome. Thus, the evaporator can be operated by the defined adjustment of the air ratio in the mixing chamber so that there defined low Tempe- training that prevents spontaneous self-ignition. Due to the low temperatures there is a slight tendency to soot, z. B. caused by cracking reactions. The mixture formation and oxidation zones are then virtually separated, so that no flame arrestor barrier is necessary. The temperatures in the evaporator are very low and virtually independent of performance, so that the thermal stress of the surrounding components is correspondingly low. The hydrocarbons can be selectively partially oxidized in a first reaction stage by means of a catalyst. Non-selective reaction paths, in particular cracking and soot formation, can thus be minimized. The use of a non-catalytic first reaction stage has the advantage that the first reaction stage can be operated at higher temperatures, since catalysts can deactivate at too high a temperature. The formation of the desired reaction products can be controlled in a targeted manner on the basis of the defined air ratio. The chosen structure forces all gaseous or possibly mixed with the gas liquid hydrocarbon molecules, unlike an open flame in combustion reactions, to flow through the packed structure and react. In combination with the high temperatures, this results in very high conversion rates and compact dimensions. The hydrogen-rich gas mixture formed in the first reaction stage can then be homogenized much more easily than the hydrocarbon-air mixture while avoiding soot, since the concentration of hydrocarbons (potential cracking candidates) is orders of magnitude smaller. In addition, homogenization can be carried out within a significantly extended operating range (at higher temperatures). ren temperatures and residence times). Subsequently, the hydrogen-rich gas mixture can be fed to a further reaction zone. Due to the homogeneity of the mixture, the corresponding reaction zone can be very compact and guarantee a uniform product gas composition. Z. B. the hydrogen-rich gas mixture can be further oxidized with low emissions. Due to the homogeneity of the mixture, the corresponding combustion / reaction chamber can be very compact.

The invention will now be described by way of example with reference to the accompanying drawings by way of particularly preferred embodiments.

Showing:

Figure 1 is a schematic representation of a first embodiment of a system for providing a homogeneous fuel-air mixture starting from liquid fuel;

FIG. 2 shows a schematic representation of a second embodiment of a system for providing a homogeneous fuel-air mixture starting from liquid fuel;

Figure 3 is a graph of temperature and power values occurring in a device according to the invention versus time; and FIG. 4. the free enthalpy of selected hydrocarbons and the elements carbon and hydrogen as a puncture of the temperature.

In the following description of the preferred embodiments of the invention, like reference characters designate like or similar components.

Figure 1 shows a schematic representation of a first embodiment of a system for providing a homogeneous fuel-air mixture starting from liquid fuel. The core of the exemplary system shown in FIG. 1 for providing a fuel-air mixture, starting from liquid fuel, is the fuel evaporator 2 arranged in an evaporator receptacle 1. Attached to the evaporator receptacle is a holding element 3b for receiving a packed structure 3, which is provided with a fiber mat 3c may be sheathed for mechanical fixation or thermal insulation. Liquid fuel or oxidizing agent are supplied to the system via the oxidant feed 4 and / or the fuel feed 5. The oxidizing agent, preferably air and optional additives such. B. Steam, enters through holes 6 radially inwardly into the mixing chamber 12, where a mixing of the oxidizing agent takes place with the in the mixing chamber 12 upstream evaporator chamber 13 vaporized fuel. In general, the evaporator chamber 13 and the mixing chamber 12 form a uniform volume, with the evaporation taking place upstream and the mixing more downstream. However, backflow can also take place in the direction of the fuel evaporator, so that even there a fuel and air. Furthermore, favored by the low temperatures on the metal fleece, the evaporation of high boilers in the evaporator chamber 13 is incomplete, so that this then takes place in the mixing chamber 12, since the fuel in the direction of example 900 ⁰ C hot packed structure 3 flows. The oxidation of the fuel-air mixture is carried out optionally by an electrical ignition element 7 or by the packed structure 3. The zone 8 following the reaction zone 14 containing the packed structure 3 serves to homogenize the resulting oxidation products. The homogenized gas mixture can then within the further reaction zone 9, z. B. by a supply of oxidizing agent via a further oxidant supply 10, are further converted. The type of feed significantly influences the mass and heat transport in the reaction zone 9. By means of radial bores 11 z. B. the reaction products transported to the wall, so that a part of the heat of reaction can be easily released into the environment; In this way it is prevented that the downstream of the reaction zone 9 components overheat.

The system for providing a fuel-air mixture can be operated at different ambient temperatures, depending on the application requirement. At very high ambient temperatures (> 400 ^° C.) there is a risk that too much heat is introduced into the fuel vaporizer and thus cracking reactions (for example in the fleece) are driven. Accordingly, generally moderate ambient temperatures are advantageous. Nevertheless, the requirements for the fuel evaporator due to the application are such, that this must be operated in a hot environment, measures for thermal insulation of the evaporator are advantageous, as will be described below. In this case, it is advantageous that the areas indicated by the reference symbols 2 a and 3 b have a low thermal conductivity or the heat conduction to the nonwoven fabric is minimized constructively. This can be minimized, for example, by the choice of thin webs in the region of the reference numeral 2a. Furthermore, it may be advantageous that between see the areas 2 and 3b, a thermal insulator, not shown in Figure 1 is provided, for example, a ceramic disc. Furthermore, a thermal insulation of the evaporator is possible and optionally attached. This applies equally to the oxidant and fuel path.

Figure 2 shows a schematic representation of a second embodiment of a system for providing a homogeneous fuel-air mixture starting from liquid fuel. In contrast to Figure 1, the supply of the oxidant stream in the second reaction zone is multi-stepped and directed radially inward.

FIG. 3 shows a graph of temperature and power values which occur in a device according to the invention as a function of time. Exemplary results are presented with such a system for diesel vaporization in air. The packed structure used contains a catalyst which partially oxidizes the die so that a hydrogen-rich gas mixture is formed. This can in a fuel cell system such. B. an auxiliary power unit (APU) for Electricity and heat generation can be used. FIG. 3 shows the measured temperatures in the evaporator chamber (curve a) and the catalyst (curve b) for thermal outputs (curve c) between 1 and 4 kW and an air ratio of 0.3 to 0.35. As can be seen, the temperature in the evaporator chambers is approx. 300 ⁰ C. Despite the high temperature in the center of the catalyst (1100 ⁰ C maximum), there is no re-ignition of the fuel-air mixture within the mixing chamber. Even with an increase in the residence time within the mixing chamber (reduction of thermal power from 4 to 1 kW) no ignition is observed, so that a stable operation can be realized.

The features of the invention disclosed in the foregoing description, in the drawings and in the claims may be essential to the realization of the invention both individually and in any combination.

List of reference numbers:

I Evaporator element 2 Fuel evaporator

2a bridge

3 packed structure

3b holding element

3c fiber mat 4 oxidant supply

5 fuel feed

6 holes

7 electrical ignition element

8 homogenization zone 9 reaction zone

10 reaction / oxidant feed

II bore

12 mixing zone / mixing chamber

13 evaporation zone / evaporation chamber 14 reaction zone

Claims

1. A device for providing a homogeneous mixture of fuel and oxidant, with

Means (5) for supplying liquid fuel to an evaporator,

Means (4) for supplying gaseous oxidant in a mixing zone downstream of the evaporator and

one of the mixing zone (12) downstream reaction zone,

characterized in that in the reaction zone (14) a packed structure (3) is arranged.

2. Apparatus according to claim 1, characterized in that the packed structure (3) is formed as a ceramic cylindrical molded body having a diameter between 25 and 35 mm and an axial length between 15 and 50 mm in length.

3. Device according to claim 1 or 2, characterized in that the packed structure (3) has in cross-section square flow channels with a cell density between 400 and 1200 cpsi.

4. Device according to one of the preceding claims, characterized in that the surface of the packed structure (3) at least partially has a coating as a catalyst.

5. Device according to one of the preceding claims, characterized in that the reaction zone (14) is a homogenization zone (8) downstream.

6. Apparatus according to claim 5, characterized in that the homogenization zone (8) is a further reaction zone (9) downstream.

7. Device according to one of the preceding claims, characterized in that it is in the mixing chamber (12) with a defined air ratio below 0, 5 operable.

8. Device according to one of the preceding claims, characterized in that it is operable in a between the evaporator (2) and the mixing zone (12) arranged evaporation zone (13) with temperatures corresponding to the boiling end of the fuel used temperatures not or only insignificantly exceed.

9. Apparatus according to claim 8, characterized in that the evaporation zone (13) and the mixing zone (12) have a volume, so that mean residence times of the fuel-oxidizer mixture are in the order of the reaction times of oxidation reactions.

10. A method of providing a homogeneous mixture of fuel and oxidant comprising the steps of:

Supplying liquid fuel to an evaporator (2),

Supplying gaseous oxidant and vaporized fuel into a mixing zone (12) downstream of the evaporator (2),

Mixing of oxidant and fuel in the mixing zone (12) and

Introducing the mixture formed in the mixing zone (12) into a reaction zone (14),

Characterized in that a packed structure (3) is arranged in the reaction zone (14) through which the entire mixture or already formed reaction products are passed.